I collected the data primarily through going each episode and recording the locations that appeared in the episode. When the location wasn't altogether clear, I would check it against the very thorough X-Files wiki. While The X-Files locations typically have real world counterparts, occasionally they use fictional towns (e.g. 'Bellefleur, Oregon' in the pilot), or open-ended locations (e.g. episode 5x5 is set 'In some midwestern town'). In these cases, some artistic license had to be taken in order to put these episodes on the map. I made it obvious within the synopses which locations were fictional or estimated.
As the series is set in Washington, DC, episodes usually begin and end there as the agents report for duty. I chose only to map an episode in DC if it is exclusively set there to avoid overcrowding and redundancy. I also placed the markers in different parts of the city to avoid total overlapping of markers, but it is important to note that the different locations within the city are irrelevant.
Additionally, to maintain a consistent level of detail throughout the maps, the pushpins are only placed in the town or county indicated in the episode. This is because even though the creators of the show give very specific locations (e.g. 3x6 'Cleveland Police Department'), many of these do not have real life counterparts (e.g. 3x19 'Bayside Funeral Home, Chinatown, San Francisco').
Most episodes have a single designated location (the town, city, or county they take place in), however, in several cases it is necessary to allocate two or three locations per episode if the agents track a case through several places. Multiple locations were only allocated if significant events took place outside of the original location of the investigation. The exception being whenever a location outside the United States was shown; those places were mapped regardless of much time the agents spent there.
I used the official X-Files episode and season synopses released with the DVDs.
Disclaimer: These characters and stories belong to Chris Carter. I am only borrowing them for a project to further my understanding of web-based mapping!